High intensity discharge (HID) lamps provide light by producing an arc between an anode and a cathode, rather than energizing a filament. These lamps include dc metal halide lamps and high pressure Xenon lamps which combine high luminance and good color retention. Applications include low dc input, portable light fixtures and ac powered fiber-optic illuminators used for industrial and medical lighting.
Typical ballast circuits take a low input, ac or dc, and amplify the input over several stages producing high currents and thus requiring circuits which can control the current to provide a more steady current thereby providing a more steady light output, i.e. no flickering of the light. However, because these circuits operate at higher currents, the circuits must include heavy duty components specially designed to carry such high currents which further adds to the expense of producing these circuits. Moreover, because the ac voltages include high frequencies, the circuits must protect against acoustic arc resonance induced by the high frequencies which further causes the light to flicker.
By requiring numerous and complex circuits to accommodate the above requirements, there is a high power loss in the form of heat due to the high current through so many different circuits. This further requires a means for cooling the circuit by a cooling interval before relighting the lamp as well as the need for actively cooling the circuits by including fans, further adding to the current consumed by the circuit to operate the fans and further increasing the power losses.
As a result, these lamps are neither energy efficient nor inexpensive to produce and operate.